Di-lower alkylacetals, such as acetaldehyde dimethylacetal, acetaldehyde diethylacetal, acetaldehyde dipropylacetal, and acetaldehyde dibutylacetal, are industrially useful compounds as, for example, intermediates for synthesizing various industrial materials, especially alkyl vinyl ethers which are useful as organic solvents, synthetic perfumes, synthetic resins, and adhesives, and N-vinylcarboxylic acid amides which are starting materials for hydrophilic polymers.
Di-lower alkylacetals are usually prepared by reacting acetaldehyde with a lower alcohol in the presence of an acid catalyst. It is generally accepted that the difference of the starting lower alcohol in kind makes no noticeable different in the mode of the unit reaction. However, as a matter of course, the kinds of the reaction product and by-product vary according to the starting alcohol. The physical properties of the acetal product which greatly influence separation and purification steps, such as distillation characteristics, crystallizability, compatibility with solvents, and distribution coefficient, largely depend on the starting material. Particularly in the production on an industrial scale, factors other than the reactivity of starting materials, for example, the cost, availability, easy handling of starting materials, the final yield of a desired product after isolation and purification, simpleness of the total production line inclusive of isolation and purification steps, the cost of the plant, ease in operation and maintenance, and the cost of utility, are also of great importance. Accordingly, the process to be adopted, inclusive of material selection, should be decided taking all these factors into due consideration.
Various proposals for the mode of process per se for preparing a di-lower alkylacetal from an aldehyde and a lower alcohol have been made to date. For example, a process of using allyl alcohol is disclosed in JP-A-3-246247 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). The reaction as disclosed being an equilibrium reaction, the result obtained is no more than the conversion at equilibrium. JP-A-62-116534 describes a process starting with an alcohol having 4 carbon atoms. This process requires a large quantity of calcium chloride as a catalyst and a dehydrating agent for achieving satisfactory reaction results. Besides, the process involves extra steps after the reaction, e.g., removal of the catalyst and washing of the reaction mixture.
Various studies have also been given to production of acetaldehyde dimethylacetal from acetaldehyde and methanol, and use of an acid catalyst, such as hydrochloric acid, sulfuric acid, an organic sulfonic acid, an inorganic solid acid (e.g., zeolite) or an ion-exchange resin is generally known, as described in U.S. Pat. Nos. 3,641,163 and 2,840,615, JP-B-62-59097 and JP-B-62-41492 (the term "JP-B" as used herein means an "examined published Japanese patent application"). Similarly to the cases of using other lower alcohols, this reaction as such has a limited conversion by nature of the equilibrium reaction. In order to increase the yield, therefore, it would be necessary to take some manipulation, for example using one of the reactants in large excess or quickly driving out the reaction product from the reaction system. Since the system after reaching to equilibrium in the reaction usually contains by-produced water and the unreacted acetaldehyde in addition to the desired acetal, distillation of the reaction mixture in which the catalyst has been neutralized or from which the catalyst has been removed tends to induce decomposition of the desired dimethylacetal or by-production of undesired impurities.
In the production of dimethylacetal, too, calcium chloride may be used as a catalyst and a dehydrating agent for removing the by-produced water to improve the conversion. However, it must be added in large quantities. This causes an extra cost for water discharge and handling complexity. Removal of by-produced water by azeotropic distillation by using an inert solvent, such as n-heptane or toluene, is also known, but the process involves an extra step for separating and recovering the solvent and cannot be regarded beneficial. While appropriate combinations of these techniques have also been proposed, the outstanding problems still remain unsolved.
In general, a reaction system called a reactive distillation process in which the reaction is carried out while conducting distillation to improve the equilibrium conversion and an apparatus therefor are known as a means for overcoming the drawbacks associated with an equilibrium reaction, such as limitation of a conversion, and there have been reported many cases for equilibrium reactions using an acid catalyst, for example, an esterification reaction (see JP-A-63-277645), an etherification reaction (see JP-A-1-316337), and an acetal reaction (see JP-B-62-29419 and JP-A-3-56134).
The reactive distillation process is considerably influenced by distillation characteristics of various substances present in the reaction system, i.e., starting compounds, the product, by-products, etc., as well as the above-mentioned problems. It is therefore especially important to decide the whole production line taking into account the kinds of the starting materials and the catalyst the reaction conditions, the reaction operation, and the isolation step.
For example, JP-B-62-29419 proposes to prepare an acetal of an unsaturated alcohol having 3 or more carbon atoms by reactive distillation using nitric acid having a relatively low boiling point as a catalyst. However, the process proposed, when applied to synthesis of acetaldehyde dimethylacetal from methanol and acetaldehyde which are the cheapest alcohol and aldehyde, it turned out substantially impractical because the desired acetal product and nitric acid both run from the top of the tower to produce an undesired high-boiling by-product.
JP-A-3-56134 proposes a reactive distillation apparatus for carrying out an equilibrium reaction using a solid acid or a solid base as a catalyst, which is characterized by forcedly circulating the reaction mixture in the reactor. This apparatus is effective where a formalin aqueous solution containing a large quantity of water, which is the main cause of catalyst deterioration, is used as a starting material, that is, in the production of methylal from methanol and a formalin aqueous solution, because of ease in frequent regeneration and exchange of the catalyst. Nevertheless, the apparatus requires additional equipment, such as a pump, which unavoidably entails the cost of construction and operation.
Where a reactive distillation system is applied to the production of acetaldehyde dimethylacetal, since the acetaldehyde dimethylacetal produced and methanol form an azeotropic mixture, a special manipulation should be taken for separation of the acetal and methanol. To this effect, JP-A-58-103331 proposes to conduct azeotropic distillation of a methanol-methylal mixed system by a combination of distillation under pressure and distillation under normal pressure or reduced pressure, utilizing the fact that the proportion of the azeotropic composition varies by changing the pressure of distillation. However, even with the pressure condition varied, the closeness of the boiling points of these two components necessitates great increases in the number of plates of the distillation tower and reflux ratio, which entails the high cost for operation and construction.
German Patent 1007311 proposes extractive distillation with water for a methanol-dimethylacetal system. Apart from difficulty in completely removing methanol, the process has the problem that the acetal obtained contains water, and the water must be removed by azeotropic distillation or with a desiccator, which is not only troublesome but accompanied with a loss in yield.
JP-B-38-19707 suggests to separate an acetal-alcohol system by extractive distillation using an alcohol or an amino compound. However, when such a reactive compound is added to the system and heated, the system suffers an unfavorable side reaction, such as decomposition of the desired product or production of impurities, only resulting in reductions in purity and yield of the product.
It is generally known that a mixture having a nearly azeotropic composition may be separated by distillation in the presence of an azeotrope former (solvent) as a third component. The azeotrope former to be added is chosen according to requirements: (1) to form an azeotrope whose azeotropic point is lower than that of a methanol-acetal azeotrope with a great difference sufficient for effective separation of the acetal, (2) to form an azeotrope having a high methanol content from the energy consideration, and (3) to be inert to the acetal. For the time being, no azeotropic former satisfying all these requirements has been suggested.